Base Analogs

Base similar to bases normally found in DNA so can be incorporated into DNA in place of normal bases since they are so similar. Exist in normal and rare tautomeric states. These mutations depend on replication

Base modifying agents

Thymine dimers

Common lesion in DNA caused by UV radiation, where adjacent thymines in same strand are linked in an abnormal way that distorts the double helix at that site. Induced mutation where covalent bonds form between thymines instead of at bottom. Reversed by UV radiation

Chemical mutagens

Intercalating Agents

Used to stain DNA in gel electrophoresis. Inserts themselves between adjacent bases in one or both strands of the DNA double helix causing the helix to relax. Can result in base-pair addition or deletion, resulting in frameshift mutation.

S9

Mismatch repair in Bacteria

In E. coli, methyl-directed mismatch repair is used when error remains in bases even after proofreading mechanism. Using this type of repair, mismatches are recognized, incorrect bases are excised, and repair synthesis is carried out. To identify erroneous base, you have to distinguish between the two strands. Usually the parental strand is the good strand. You can identify parental strand by finding the methylated A nucleotide at the beginning of the sequence. Nucleotides should be methylated but it takes sometime after replication for methylation to occur. Once unmethylated, erroneous nucleotide found, excision and repair can occur.

Missense Mutation

Nonsense Mutation

Gene mutation where a base-pair change in the DNA changes an mRNA codon for an amino acid to a stop codon. Instead of changing outcome, new mRNA codon is nonsense and codes for nothing. Causes premature chain termination.

Silent Mutation

Frameshift Mutation

When one or more base pairs are added or deleted causing the reading frame to shift. mRNA's move by one base so that incorrect amino acids are added to the polypeptide chain after mutation site. Can result in a nonfunctional protein. May generate new stop codons, resulting in shortened protein. May result in longer proteins from reading through stop codons. Worse when toward start.

Transition Mutation

Reverse Mutation

Change genotype from mutant to wild-type or partially wild-type. Reversion of nonsense mutation occurs when a base-pair change results in a change of the mRNA nonsense codon to a codon for an amino acid.

True Reversion

Partial Reversion

Supressor Mutation

Class of Point mutation. Mutation at a different site from that of original mutation. Effects of a mutation can be diminished. Masks or compensates for effects of initial mutation but doesn't reverse original mutation. May occur within same gene or not.

Suppressor Genes

Spontaneous Mutation

Occurs naturally at DNA replication, G1 phase, or G2 phase. All types of point mutations can occur spontaneously. Most corrected by cellular repairs. Occurs at a rate of 10 -4 and 4x10 -6 per gene per generation.